Notices of Memoirs — P. H. Carpenter on the Blastoidea. 465 



It is very doubtful whether the genus Penlremites occurs at all in 

 Britain. Some badly-preserved fragments from the Devonian and 

 the Scotch Carboniferous are possibly referable to it ; but most of 

 the Blastoids (besides Cadaster) which occur in the Carboniferous 

 Limestone belong to the genus Granatocrinus, Troost., which is 

 represented by some seven or eight species. 



Cumberland's Mitra eUiptica is the representative of a new genus, 

 distinguished by the eccentric position of the spiracles. Cadaster is 

 a true Blastoid, and not a Cystid, as supposed by Billings. The slit- 

 like openings of its hydrospires are nearly on the same level as the 

 ambulacra, which do not conceal them at all. In the ordinary 

 Blastoids, however, they are below and concealed by the ambulacra, 

 opening externally by pores at the sides of the latter. There are 

 various intermediate forms between these two extremes, in which 

 the hj^drospiral slits are more or less concealed by the ambulacra, 

 but are partially visible at their sides. It is proposed to group the 

 species thus distinguished into a genus Pentremitidea, which is repre- 

 sented in Britain by the little Pentremites acutus, Sowerby, in Belgium 

 by P. caryophyllatus, and in Spain by P. Pailleti, De Verneuil, for 

 which last the name Pentremitidea had been already proposed by 

 D'Orbigny. An arrangement of this kind has been already suggested 

 by Billings. 



The discoveries of Rofe, Wachsmuth, and Hambach, respecting the 

 perforation of the lancet-piece by a longitudinal canal, are confirmed. 

 This canal probably lodged the water-vessel, which must have been 

 devoid of any tentacular extensions, as in some Holothurians, and in 

 the arms of certain Comatidce. Respiration was efi'ected, however, by 

 means of the hydrospires. The pores usually found at the sides of 

 the ambulacra were not the sockets for the attachment of the 

 appendages, but led downwards into the hydrospires, serving to 

 introduce water, which made its way out through the spiracles. The 

 genital ducts probably opened into some portion of the hydrospires, 

 as they do into the closely similar structures of the OpMuroidea, and 

 the ova were discharged through the spiracles. Billings' statements 

 are confirmed respecting the existence in many species of a single or 

 possibly double row of joined appendages along each side of the 

 ambulacra ; but these appendages are not homologous with the 

 pinnules of the Crinoidea. 



In perfect specimens the peristome is covered in by a vault of 

 small polygonal plates, any definite arrangement of which is rarely 

 traceable. Extensions of this vault were continued down the sides 

 of the ambulacral grooves, which could thus be closed in completely 

 and converted into tunnels, as in recent Crinoids. 



The classification of the Blastoidea must depend entirely upon 

 morphological principles. Mere differences in the relative sizes of 

 the calyx plates are of very little systematic value ; and differences 

 in the numbers of side plates on given lengths of the ambulacra are 

 absolutely worthless. On the other hand, the structure and relative 

 positions of the hydrospires and spiracles are morphological characters 

 of much systematic value. 



DECADE n. — VOL. VIII. — NO. X. 30 



